Requirements of Optimal Synthetic Aperture Radar (SAR) Frequency, Polarization and
Incidence Angle for Mapping Underwater Bottom Topography: A Simulation Study
Method of solution
The calculation of surface currents from
equations (1)-(3) has been reduced to a
quasi-one-dimensional problem since the
underwater bottom topography in fig.1 is
perpendicular to the current direction. It is
also assumed that the current flow is laminar,
free of vertical current shear and quasi-stationary.
Thus, the current
(x) is derived
from the simple continuity equation. The
action balance equation (4) is solved
numerically following the method of
characteristics employed by Hughes (1978).
The normalized radar backscattering cross
section (
s 0) is finally calculated from
equations (7) and (8).
Results
Optimal radar frequency
Fig.2 is an example of the simulation for
optimal radar frequency showing the
variations of
s 0 with underwater bottom
topography for
q=23° , d=10m , h
1=10m , V V
polarized P, L, C and X bands.
D s 0=
½ s 0
-
s 00½ , s 00
being the normalized
radar backscattering cross section at x=0 .
It can be seen from fig.2(a) that
s 0 for all
bands decreases slowly with increase of the
height of the sandwave and reaches their
minimum near the crest at x=573m. The
s 0
then increases rapidly with decrease of
the height of the sandwave.
s 0p(
s 0
for P band) has a largest variation while
s 0x
(
s 0 for X band) has a smallest variation across
the sandwave. This means that long
wavelength radars can see the pattern of the
sandwave more clearly than the short
wavelength radars. It can be found from
fig.2(b) that for each location of the
sandwave
½ D s
0P½>
½ D s 0 L½>
½ D s 0C½
>
½ D s 0x½.
This indicates that underwater bottom
topography can be detected more easily by
the longer wavelength radars. Simulation for
other cases shows similar results. It can be
concluded from our simulation that long
wavelengths are required. P band is the
optimal band for mapping underwater
bottom topography, followed by L, C and X
bands.
Optimal polarization
Our simulation results show that
s 0
for P, L,
C and X bands do not depend on the
polarization. However,
s 0 from the sea
surface for V V polarization is highest,
which yield the best signal-to-noise ratio.
Thus, V V polarization is to be preferred for
mapping underwater bottom topography.
Fig.2 Variations of s 0 (a) and D
s 0 (b) with
underwater bottom topography(c).
Optimal incidence angle
Fig.3 depicts the relationship between the
incidence angles and the
s 0 for d=7m,
h
1=10m, V V polarized P, L, C and X bands.
It can be seen that the
s 0 decreases with
incidence angles. The signal from the sea
surface is too small to be detected by
spaceborne radar when the incidence angle
is very large. Compared with the Radarsat
SAR noise equivalent sigma naught of –18.5
dB (minimum detectable signal) (parashar,
et al., 1993), the optimal range of the
incidence angle for mapping underwater
bottom topography is between 20°
to 40°.
Conclusions
In this work the radar backscattering cross
section of the sea surface has been simulated
and analyzed. Form the results of the
simulation the following conclusions are
drawn. For mapping underwater bottom
topography, large wavelengths (P and L
bands), V V polarization and small incidence
angles (20°
to 40°) are prefered.
Fig.3 Variations of s 0 at
P((a)),L((b)),C((c)) and X((d)) bands
with the radar incidence angles
Acknowledgments
This work was supported by the China 863
Program under the Projects 818-06-02 and
2-7-4-15.
References
- Alpers, W. and Hennings, I., 1984, A theory
of the imaging mechanism of underwater
bottom topography by real and synthetic
aperture radar. Journal of Geophysical
Research, 89C, 10529-10546.
-
de Loor, G. P., 1981, The observation of
tidal patterns, currents and bathymetry
with SLAR imagery of the sea. I.E.E.E
Journal of Oceanic Engineering, 6, 124-129.
-
Hughes, B. A., 1978, The effect of internal
waves on surface waves. 2. Theoretical
analysis, Journal of Geophysical
Research, 83C, 455-465.
-
Parashar, S., et al., 1993, Radarsat mission
requirements and concept. Canadian
Journal of Remote Sensing, 19, 280-288.
-
Schmullius, C. C. and Evans, D. L., 1997,
Synthetic aperture radar (SAR)
frequency and polarization requirements
for applications in ecology, geology,
hydrology and oceanography: a tabular
status quo after SIR-C/X-SAR. Int. J.
Remote Sensing, 18, 2713-2722.
